Conventional Gas Carburization
Traditional (high temperature) carburization is a widely used industrial process for enhancing the surface hardness of shaped metal articles (“case hardening”). In a typical commercial process, the workpiece is contacted with a carbon containing gas at elevated temperature whereby carbon atoms liberated by decomposition of the gas diffuse into the workpiece's surface. Hardening occurs through the reaction of these diffused carbon atoms with one or more metals in the workpiece thereby forming distinct chemical compounds, i.e., carbides, followed by precipitation of these carbides as discrete, extremely hard, crystalline particles in the metal matrix forming the workpiece's surface. See, Stickels, “Gas Carburizing”, pp 312 to 324, Volume 4, ASM Handbook, ©1991, ASM International.
In the last few years, new methods have been introduced for carrying out traditional carburization in which acetylene supplied at very low pressures is used as the carburizing gas. A primary benefit claimed for this approach is that the amount of by-product soot that is foamed as part of the carburization reaction is reduced. See, EP 818 555 and corresponding U.S. Pat. No. 5,702,540. In some instances, acetylene flow to the reaction chamber is pulsed rather than constant, as this is said to reduce soot formation even further.
Stainless steel is corrosion-resistant because of the coherent, impervious layer of chromium oxide which inherently forms on the surface of the steel as soon as it is exposed to the atmosphere. When stainless steel is traditionally carburized, the chromium content of the steel is depleted through the formation of the carbide precipitates responsible for surface hardening. As a result, there is insufficient chromium in the steel, at least in areas immediately surrounding the chromium carbide precipitates, to form this chromium oxide protective coating. For this reason, stainless steel is rarely case hardened by conventional carburization, since the corrosion resistance of the steel is compromised.
Low Temperature Gas Carburization
In the mid 1980's, a technique for case hardening stainless steel was developed in which the workpiece is contacted with a carbon containing gas at low temperature, typically below ˜550° C. (˜1000° F.). At these temperatures, and provided that carburization does not last too long, carbon atoms liberated by decomposition of the gas diffuse into the workpiece surfaces, typically to a depth of 20-50 μ, without formation of carbide precipitates. Nonetheless, an extraordinarily hard case (surface layer) is obtained. Because carbide precipitates are not produced, the corrosion resistance of the steel is unimpaired, even improved. This technique, which is referred to a “low temperature carburization,” is described in a number of publications including U.S. Pat. Nos. 5,556,483, 5,593,510, 5,792,282, 6,165,597, 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853).
Original thinking was that surface hardening occurs in low temperature carburization due solely to the stress placed on the crystal lattice of the metal by the carbon atoms which have diffused into this lattice. However, recent analytical work suggests that an additional phase or phases may be formed in this hardened surface layer. While the exact nature of these additional phases is still unknown, what is known is that the chromium content of these additional phases is identical to that of the surrounding metal matrix. The result is that the corrosion resistance of the steel remains unimpaired, because the chromium responsible for corrosion resistance remains uniformly distributed throughout the metal.
Acetylene
U.S. Pat. No. 7,122,086 to Tanaka et al., the entire disclosure of which is also incorporated herein by reference, describes a low temperature gas carburization process in which a stainless steel workpiece is carburized by contact with acetylene in a hard vacuum, i.e., at a total pressures of 1 torr (133 Pa (Pascals)) or less. A primary benefit claimed for this approach is that the production of soot and undesirable thermal oxide film byproducts is substantially reduced. Nonetheless, the carburized workpiece obtained still needs to be treated, mechanically and/or chemically, to remove these byproduct layers before a usable, final product is obtained.
WO 2006/136166 (U.S. 2009/0178733) to Marcel Somers et al., the entire disclosure of which is also incorporated herein by reference, describes a similar low temperature gas carburization process in which acetylene is used as the carbon source for the carburization of stainless steel workpieces. Both atmospheric and subatmospheric pressures are disclosed. If desired, hydrogen (H2) can be included in the carburizing gas to facilitate decomposition of the acetylene and make control of the process easier.
Activation
As indicated above, stainless steel is corrosion-resistant because of the coherent, impervious layer of chromium oxide which inherently forms on the surface of the steel as soon as it is exposed to the atmosphere. Because the temperatures involved in low temperature carburization are so low, carbon atoms will not penetrate this chromium oxide protective coating. Therefore, low temperature carburization of stainless steel is normally preceded by an activation step in which the workpiece is contacted with a halogen containing activating compound such as HF, HCl, NF3, F2 or Cl2 at elevated temperature, e.g., 200 to 400° C., to make the steel's protective oxide coating permeable to carbon atoms. See, the above-noted U.S. Pat. Nos. 5,556,483, 5,593,510, 5,792,282, 6,165,597, 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364), Japan 9-71853 (Kokai 9-71853), and U.S. Pat. No. 7,122,086 to Tanaka et al.
See, also, the above-noted WO 2006/136166 (U.S. 2009/0178733) to Marcel Somers et al., which indicates that such a separate activation step is unnecessary if acetylene is used as the carburizing gas, as decomposition of the acetylene for carburization also activates the chromium oxide coating. In practice, however, activation this way is unsuitable for commercial operations, because carburization is too slow, the results obtained too uneven, or both.
Clean UP
Low temperature gas carburization normally produces soot as an unwanted by-product. In addition, low temperature carburization also produces an undesirable, porous “thermal” oxide film on the outermost surfaces of the workpiece about 20-30 nm thick. See, Japan 9-71853 (Kokai 9-71853). In addition, under this thermal oxide film, an extremely thin outer surface layer of the metal may contain a small amount of carbide precipitates, especially if the low temperature carburization conditions are too severe. See, U.S. Pat. Nos. 5,556,483, 5,593,510 and 5,792,282. In order for the workpiece to exhibit an attractive shiny, metallic appearance, this soot and outermost thermal oxide film must be removed. Therefore, as a practical matter, these undesirable surface layers (i.e., the soot, thermal oxide film, and thin outermost metal layer containing carbide precipitates, if any) are removed before the workpiece is used. Normally, only a minimal amount of the workpiece's metal surface is removed, about 1 micron or so, since the hardened “case” produced by low temperature carburization only extends down to the first 10-25 microns or so of the workpiece's surface.
In any event, in the context of this disclosure, reference to a workpiece surface layer which is “essentially free of carbide precipitates” or which is made “without formation of carbide precipitates” refers to the corrosion-resistant, carbon-hardened surface layer underneath these unwanted by-product layers. For convenience, this corrosion-resistant, hardened byproduct-free surface layer is referred to herein as the “primary” surface layer of the workpiece.
In our earlier published application U.S. 2011/0030849, the disclosure of which is also incorporated herein by reference in its entirety, we describe a process for the low temperature gas carburization of stainless steel which is carried out without formation of the above-noted soot and thermal oxide film. This is done by carrying out the carburization reaction in a soft vacuum, i.e., a total reaction pressure of about 3.5 to 100 torr (˜500 to ˜13,000 Pa), using acetylene or analog as the carburizing gas. Separate activation by contact with a halogen containing gas is still required, as a practical matter, for the reasons indicated above, i.e., because carburization is too slow, or the results obtained too uneven, if activation occurs solely through decomposition of the acetylene.